http://www.rc-monster.com/proddetail...540C_XL&cat=20
Mike has 7,8,9XLs.
i don't think there is anyway to get a MT to 50 mph on 14 cells and be a reliable setup. Maybe if you had a super light truck, but for the avg 10lb'er, no.
A 7xl should get you in the ~lower mid 40s tho, and will be quite fast and very TQ'y. To go anymore than that you will need more cells.